The present application incorporates by reference U.S. Pat. No. 5,654,755, assigned to the assignee hereof.
The present invention relates to image sensor arrays used in raster input scanners. In particular, the invention relates to photosensitive chips wherein each photosensor outputs signals onto a common video line.
Image sensor arrays typically comprise a linear array of photosensors which raster scan an image bearing document and convert light from the image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors.
For high-performance image sensor arrays, a preferred design includes an array of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging without reductive optics. In order to provide such a xe2x80x9cfull-widthxe2x80x9d array, relatively large silicon structures must be used to define the large number of photosensors. A preferred technique to create such a large array is to make the array out of several butted silicon chips. In one proposed design, an array is intended to be made of 20 silicon chips, butted end-to-end, each chip having 248 active photosensors spaced at 400 photosensors per inch.
Photosensitive devices may be one-dimensional or two-dimensional, and can be either of the xe2x80x9cactivexe2x80x9d variety, wherein the photosensors output voltage signals, or in the form of a charge-coupled device, or CCD, which outputs a sequence of charges from a series of individual photosensors. In all of these various types of photosensitive devices, a common design feature is the use of xe2x80x9cdarkxe2x80x9d photosensors, which are used to periodically reset the offset voltage for the photosensors being read out. These dark photosensors are of the same semiconductor structure as the other xe2x80x9cactivexe2x80x9d photosensors on each chip, but the dark photosensors are not exposed to light. In most designs, the dark photosensors are provided with an opaque shield, such as of aluminum or silicon, to prevent the influence of light thereon. In the scanning process, with each readout cycle of active photosensors on each chip, the readout of the first photosensor is proceeded by readouts of one or more dark photosensors, which are used to reset the voltage offset associated with the whole chip, and thereby correct signal drift when the active photosensors are reading out their signals. In other words, the readout of a dark photosensor with each scan can serve as a reference offset or xe2x80x9czero pointxe2x80x9d so that the absolute values of light intensity on the active photosensors may be determined. The use of a dark photosensor output when reading out signals from active photosensors can significantly compensate for performance variations of multiple chips in a single apparatus, and also for changes in the performance of a photosensitive device over time.
According to certain designs of scanning apparatus, certain sets of photosensors within a larger apparatus are occasionally temporarily inactivated. There may be various reasons for temporarily inactivating certain photosensors within a larger scanner. For example, in a full-color apparatus in which different sets of photosensors are respectively sensitive to different primary colors, sets of photosensors which are sensitive to one particular color may be temporarily inactivated while photosensors for another color are being used; also, in large-scale, engineering-type input scanners, it may be desirable to disable certain portions of a long array of photosensors, such as when it is known that original documents of a relatively small size are to be scanned. In these situations where certain photosensors within a larger apparatus are not used, the inactivated photosensors themselves will still function as photosensors (in the sense of generating charges in response to light impinging thereon) but the resulting voltage signals are not read out of the photosensitive chip. This situation causes a practical problem, in that the electrical activities of the unused photosensors can cause the offset controls associated with the video line to drift or otherwise become unpredictable. It is an object of the present invention to provide a photosensitive apparatus in which the fact that certain photosensors are temporarily disabled does not affect the offset control of the apparatus.
U.S. Pat. No. 5,654,755 describes a circuit for correcting the offset of the video output of a set of active photosensors, based on the output of dark photosensors. An averaging RC circuit in parallel with the video line accumulates an average signal based on a large number of readings from the dark photosensors. The average signal is periodically clamped to a correction capacitor in series on the video line, the charged correction capacitor adjusting the offset on the active-photosensor signals which subsequently pass through the video line.
The co-pending U.S. patent application cross-referenced above is directed toward an offset restore system for use in a photosensitive device of a design similar to that shown in the ""755 patent. The co-pending application is concerned with placing an initial charge on a correction capacitor which is used to establish an off-set on the video line. The correction capacitor is xe2x80x9cjump startedxe2x80x9d with an initial pre-determined charge so that, at power up, the apparatus does not have to develop a charge on the correction capacitor. Without such a jump-start, the initial charge on the correction capacitor would be zero, and this zero charge would in fact have a distorting effect on the offset control.
According to the present invention, there is provided a photosensitive device and a method of using thereof. The device comprises a set of photosensors and a video line, the photosensors having an integration period in response to an integration signal and reading out video signals onto the video line in response to a readout signal. An offset on the video line is reset when the device receives a predetermined number of integration signals without receiving a readout signal.